diff -r 955084b4f74f -r 7d9bd50afe3d mercurial/metadata.py
--- a/mercurial/metadata.py	Thu Aug 01 13:04:38 2024 +0100
+++ b/mercurial/metadata.py	Thu Aug 01 11:14:58 2024 +0100
@@ -243,7 +243,7 @@
         return _process_linear(p1, ctx)
     elif p1.rev() == nullrev and p2.rev() != nullrev:
         # In the wild, one can encounter changeset where p1 is null but p2 is not
-        return _process_linear(p1, ctx, parent=2)
+        return _process_linear(p2, ctx, parent=2)
     elif p1.rev() == p2.rev():
         # In the wild, one can encounter such "non-merge"
         return _process_linear(p1, ctx)